. . .wife is very sensitive to mosquito bites and ends up scratching them until they bleed when she is bit.
Not a repellent, but a home remedy to stop the itching from mosquito bites, recommended to me by an old pharmacist back in the 60's: take cloudy ammonia, mix 50/50 with plain water, apply to bite with cotton ball or tissue until the itching stops. Do not use on broken/bleeding skin. Usually will stop itching for several hours. Reapply as necessary. Always and still works for me. YMMV.